For many applications, polyethylene with enhanced toughness, strength, and environmental stress cracking resistance is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processibility of the resin usually decreases. By providing a polymer with a broad or bimodal molecular weight distribution, the properties characteristic of high molecular weight resins are retained and processibility, particularly extrudability, is improved. A bimodal molecular weight distribution can be explained as follows: in a traditional molecular weight distribution plot (by size exclusion chromatography) of concentrations of species of specific molecular weight vs. log molecular weight, a multimodal molecular weight distribution would show at least two maxima, two maxima being the characteristic of bimodal. The maxima need not be equivalent in magnitude or widely separated. A broad molecular weight distribution is a representation of a similar area under the plot without the clear presence of two maxima.
Three major strategies have been proposed for the production of polyethylene resins with a broad or bimodal molecular weight distribution. One is post reactor or melt blending, which suffers from the disadvantages brought on by the requirement of complete homogenization and attendant high cost. A second is through the use of multistaged reactors, which raises questions of efficiency and, again, cost. The third, and most desirable strategy, is the direct production of a broad or bimodal polyethylene via a single catalyst or catalyst mixture in a single reactor. Such a process would provide the component resin portions of the molecular weight distribution system simultaneously in situ, the resin particles being intimately mixed on the subparticle level.
In U.S. Pat. No. 4,918,038, there is described a single reactor catalytic process for the production of polyethylene resin having a broad and/or bimodal molecular weight distribution. That process utilizes a mixed catalyst system comprising:
(a) the reaction product of
(i) a vanadium halide having the formula EQU VX.sub.3 PA1 wherein X is chlorine, bromine, or iodine and each X is alike or different; PA1 (ii) a modifier having the formula EQU BX.sub.3 or AlR.sub.(3-a) X.sub.a PA1 wherein X is as defined above; R is an alkyl radical having 1 to 14 carbon atoms; each R is alike or different; and a is 0, 1, or 2 and PA1 (iii) an electron donor, which is a liquid Lewis base in which the vanadium halide and modifier are soluble; PA1 (i) a complex having the formula EQU ZrMg.sub.b X.sub.c (ED).sub.d PA1 wherein X is as defined above; ED is an electron donor, which is liquid Lewis base in which the precursors of the complex are soluble; b is a number from 1 to 3; c is a positive number equal to or less than 4+2b; and d is a number from 4 to 10; or PA1 (ii) a vanadium oxy compound having the formula EQU VOX.sub.3, VOX.sub.2, VOX, or VO.sub.2 X PA1 wherein X is as defined above, or EQU VO(OR).sub.3 PA1 wherein R is a monovalent hydrocarbon radical having 2 to 10 carbon atoms and each R can be alike or different, wherein the vanadium halide and the vanadium oxy compound are supported; PA1 if the difference in hydrogen response between the two component catalysts is very large, then the polymer produced by the mixed catalyst system will have a bimodal molecular weight distribution; but PA1 if the difference in hydrogen response between the component catalysts is large, but not sufficient to produce a product with a bimodal molecular weight distribution, the mixed catalyst system will yield a product with a higher concentration of polymer chains above 500,000 molecular weight than is typically observed for a broad molecular weight distribution product of similar melt index. PA1 (a) sequentially impregnating an active carrier (support) material with a liquid compound which is or contains a vanadium.sup.(+3 and higher) compound followed by the reduction on the support of the vanadium compound by the deposition of a liquid reducing agent and effecting the formation of a reduced vanadium.sup.(&lt;3) compound on the support surfaces or the provision otherwise of such reduced vanadium.sup.(&lt;3) compound on the support surfaces, PA1 (b) depositing a liquid zirconium organooxy compound onto the support, PA1 (c) providing an electron donor compound for the intercomplexation of the vanadium and zirconium compounds on the support surfaces, PA1 (d) drying the sequentially impregnated support to form a flowing powder, and PA1 (e) impregnating the dried sequentially impregnated support with a Group 13 element-containing activating composition or compound. PA1 (a) independently reducing a liquid vanadium.sup.(+3 and higher) to a vanadium.sup.(&lt;3) compound, PA1 (b) forming a liquid mixture of the vanadium.sup.(&lt;3) compound with a zirconium organooxy compound, which mixture contains an electron donor compound for the intercomplexation of the vanadium and zirconium in the mixture, PA1 (c) coincidentally impregnating the support with the liquid mixture of (ii)(b), PA1 (d) drying the support to form a flowing powder, and PA1 (e) impregnating the dried coincidentally impregnated support with the Group 13 element-containing activating composition or compound. PA1 a. a catalyst composition comprising a reduced vanadium compound and a zirconium organooxy compound codeposited on an active carrier material and complexed with an electron donor material, and treated with a Group 13 element activating composition or compound, PA1 b. an aluminum alkyl cocatalyst, and PA1 c. a halogenated organic promoter, PA1 a. a reduced vanadium compound and a zirconium organooxy compound codeposited on an active carrier material and complexed with an electron donor material, and treated with a Group 13 element activating composition or compound, PA1 b. an aluminum alkyl cocatalyst, and PA1 c. a halogenated organic promoter, PA1 straight chain acyclic dienes such as: 1,4-hexadiene, 1,6-octadiene, and the like; PA1 branched chain acyclic dienes such as: 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, 3,7-dimethyl-1,7-octadiene and the mixed isomers of dihydro-myrcene, dihydroocinene, and the like; PA1 single ring alicyclic dienes such as: 1,4-cyclohexadiene, 1,5-cyclooctadiene, 1,5-cyclododecadiene, and the like; PA1 multi-ring alicyclic fused and bridged ring dienes such as: tetrahydroindene, methyltetrahydroindene, dicyclopentadiene, bicyclo(2,2,1)-hepta-2,5-diene, alkenyl, alkylidene, cycloalkenyl and cycloalkylidene norbornenes such as 5-methylene-2-norbornene (MNB), 5-ethylidene-2-norbornene (ENB), 5-propyl-2-norbornene, 5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexylidene-2-norbornene, and the like.
(b) one of the following:
(c) a halocarbon promoter having the formula EQU R.sub.e CX.sub.(4-e)
wherein R is hydrogen or an unsubstituted or halo substituted alkyl radical having 1 to 6 carbon atoms; each R is alike or different; X is chlorine, bromine, iodine, or fluorine; each X is alike or different; and e is 0, 1, or 2 provided that, if no fluorine is present, e is 2.
According to said patent,
an advantage of the process is the ability to control the molecular weight distribution of the resulting polyethylene;
the mixed catalyst system used in the process is a mixture of two or more component catalysts, each having a different hydrogen response; therefore
Beran et al., U.S. Pat. No. 4,508,842, patented Apr. 2, 1985, describe an ethylene polymerization catalyst comprising a supported precursor of vanadium trihalide/electron donor complex and alkylaluminum or boron halides, when combined with alkylaluminum cocatalyst and alkyl halide promoter, provides enhanced polymerization and productivity plus a superior polyethylene product.
Beran et al. polymerizes ethylene with or without at least one C.sub.3 to C.sub.10 alpha-olefin monomer in the gas phase at a temperature between about 30.degree. C. to about 115.degree. C. wherein the monomers are contacted with a catalyst composition comprising a supported precursor vanadium compounds and aluminum alkyl containing modifiers which are impregnated on a solid, inert carrier. The catalysts utilized by Beran et al. differentiate in comprising a supported precursor, a cocatalyst and a promoter in which the supported precursor is a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium trihalide is vanadium trichloride, VCl.sub.3. The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, including particularly tetrahydrofuran. Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used.
There is substantial literature indicating the creation of a catalytically active vanadium by the reduction of vanadium halides to the divalent state. Carrick et al., JACS, vol. 82, p. 1502 (1960) describe the reduction of VCl.sub.4 to the divalent state form of the vanadium ethylene catalyst utilizing the conventional reducing agents, such as triisobutylaluminum and zinc alkyls. Karol et al., JACS, vol 83, pp. 2654-2658 (1961) discusses the partial and total reduction of vanadium halides such as VCl.sub.4 to divalent structures and the catalytic activity resulting with respect to the polymerization of ethylene to polyethylene.
Jacob et al., Z. anorg. allg. Chem., 427, pp. 75-84 (1976) illustrate the complexity of such reduction reactions with the presence of THF. From the teachings of Beran et al., the resulting divalent vanadium compounds are complexes which include THF in the structure.
Cumulative to the above, Smith et al., U.S. Pat. No. 4,559,318, patented Dec. 17, 1985, describe a number of procedures for making VX.sub.2, where X is halogen, which involves the reduction of VX.sub.4 or VX.sub.3 by reaction with reducing agents followed by the complexation of the VX.sub.2 with an ether such as THF. Such is provided on a support surface.